Aircraft turbofan engines involve a structure wherein a large fan toward the front of the engine supplies most of the thrust for the aircraft; and the air from the fan is directed through a shell-like passageway which extends around the engine. Some portion of the input air passing through the fan is directed to the inner turbine section where fuel is burned, with the expanding gases driving a power generating turbine section. The input fan is driven through gearing and central drive shaft arrangements from the power turbine section. Assuming a constant pressure at sea level, for example, then the maximum permissible fan speed is a linear function of temperature, with a positive slope. In this connection, as the temperature increases, the air thins out and its density and oxygen content decreases. Accordingly, the maximum permissible fan rotational speed increases. Similarly, at a given temperature, at higher altitudes or lower pressure, the air is thinner, and a similar increase in the maximum permissible fan speed occurs. Another factor which comes into play is the maximum permissible speed for the fan, regardless of the physical conditions such as pressure and temperature. Also, above a certain temperature, such as 10 or 15 degrees below zero centigrade, the maximum permissible fan speed starts to decrease, with increasing temperature. This additional characteristic, therefore, has a negative slope. As one further complicating factor, when subsidiary systems such as the anti-ice equipments are turned on, this has the effect of further limiting and reducing the maximum fan speed permissible with increasing temperature.
These maximum fan speed limitations are included in instructions for pilots to be followed in the operation of the aircraft, but they are so complex as to be very demanding of the pilot's attention, and distract the pilot from other flight control matters to which his attention should be directed.
Accordingly, a principal object of the present invention is to develop an electronic system which will automatically take into consideration all of the various characteristics discussed hereinabove, which will automatically shift from the positive speed versus temperature characteristics with constant pressure, over to the negative sloped maximum speeds versus temperature characteristics, and also automatically shift from one characteristic to another when the anti-ice systems are switched on and off. In addition, this system will provide an automatic maximum limitation of fan speed regardless of pressure and temperature conditions.